When a new document library is created, several file types, or
content types, are available when you create a new document from the
SharePoint 2013 menu. You can choose an empty Microsoft Word document,
a Microsoft Excel workbook, a Microsoft PowerPoint presentation, a
Microsoft OneNote notebook, or an Excel Survey. You can also select a
New Folder.

Instead of creating a commonly used format from scratch, or copying
an existing file and then modifying it, with SharePoint, you can
customize document defaults and document templates that are available
in your library.

Modify the document template for a document library

On the Quick Launch bar, select a document library or click Site Contents to locate a library.

On the ribbon, click the Library tab.

In the Settings group, click Library Settings.

On the Library Settings page, in the General Settings section, click Advanced Settings.

On the Advanced Settings page, in the Document Template section, click Edit Template.

Tip

There can be only one default New Document. Create additional content types to use multiple templates.

Make your desired changes to the template.

Save the template.

Close the application.

Click OK.

Modify the document template for a content type

In the upper-right corner of the window, click the Settings icon.

On the menu that appears, click Site Settings.

On the Site Settings page, in the Web Designer Galleries section, click Site Content Types.

On the Site Content Types page, in the Document Content Types section, click Document.

On the Site Content Type page, click Advanced Settings.

On the Advanced Settings page, in the Document Template section,
click either the Enter The URL Of An Existing Document Template option
or the Upload A New Document Template option.

Click OK.

Caution

Changing an existing content type applies that change to that
content type across the entire site collection. Creating and
customizing a new content type is recommended.